Chemically cross-linked cellulosic fiber and method of making same

ABSTRACT

The present invention relates to cross-linked cellulosic fiber having improved absorbency characteristics when compared to conventional cross-linked fibers. The cross-linked cellulosic fiber is obtainable by reacting pulp in the sheet or fluff form with one or more reagents selected from organic molecule having acid and aldehyde functional groups “acid aldehydes.” The invention also relates to a method of producing the cross-linked fiber. The method includes heating the treated cellulosic fibers to promote intrafiber cross-linking. The cross-linked fibers are characterized by having an improved centrifuge retention capacity, fluid acquisition rate, resiliency, absorbent capacity, absorbency under load, and other absorbent properties. The inventive cross-linked fibers are useful in forming absorbent composites, and in particular absorbent cores for use in absorbent articles.

FIELD OF THE INVENTION

The present invention relates to cross-linked cellulosic fiber,obtainable by reacting pulp in sheet or fluff form with one or morereagents selected from organic molecules having acid and aldehyde groups(e.g., acid aldehydes). The invention also relates to a method ofproducing the cross-linked fiber. The cross-linked fibers of the presentinvention are characterized by having an improved acquisition rate,softness, resiliency, absorbent capacity, centrifuge retention capacity,free swell, and absorbency under load.

DESCRIPTION OF RELATED ART

Absorbent articles intended for personal care, such as adult incontinentpads, feminine care products, and infant diapers typically are comprisedof at least a top sheet, a back sheet, an absorbent core disposedbetween the top sheet and back sheet, and an acquisition layer betweenthe top sheet and the absorbent core. The acquisition layer comprisedof, for example, acquisition fibers, usually is incorporated in theabsorbent articles to provide better distribution of liquid, increasethe rate of liquid absorption, and reduce gel blocking. A wide varietyof acquisition fibers are known in the art. Included among these aresynthetic fibers, a composite of cellulosic fibers and synthetic fibers,and cross-linked cellulosic fiber. Cross-linked cellulose fiber ispreferred because it is abundant, it is biodegradable, and it isrelatively inexpensive.

Cross-linked cellulose fibers and processes of making them have beendescribed in the literature for many years (see, for example G. C.Tesoro, Cross-Linking of Cellulose, in Handbook of Fiber Science andtechnology, Vol II, M. Lewis and S. B. Sello eds. pp 1-46, MercelDecker, New York (1993)). The cross-linked cellulose fibers typicallyare made by reacting cellulose with multifunctional agents that arecapable of reacting with the hydroxyl groups of the anhydroglucoserepeating units of the cellulose either in the same chain, or inneighboring chains simultaneously. Cross-linked cellulose fibersgenerally are characterized by their high absorbent capacity, and theirhigh resiliency in the wet and dry states.

Many cross-linking agents have been discovered since the development ofcross-linked cellulose fibers. Formaldehyde and urea-formaldehydeproducts were among the first agents used to cross-link cellulosicfibers. Such cross-linking agents are disclosed in, for example, U.S.Pat. Nos. 2,764,573; 3,260,565; 3,112,156; 3,224,926; 3,241,533;3,932,209; 3,756,913; 4,035,147; and 5,225,047, the disclosures of whichare incorporated by reference herein in their entirety.

Safety problems have arisen from the use of formaldehyde andurea-formaldehyde products as cross-linking agents. These problems havecreated a need for non-formaldehyde-containing cross-linking agents toreplace the formaldehyde and urea-formaldehyde cross-linking agents. Anumber of new cross-linking agents for cellulosic fibers have becomeavailable in light of this need.

Monomers having multifunctional groups, such as carboxylic acid groupsand aldehyde groups have been widely used as cross-linking agents forcellulosic fibers. For example, alkanepolycarboxylic acids are capableof cross-linking cellulose fibers by forming an ester bond with thefiber's hydroxyl groups. Cross-linking can occur upon heating thealkanepolycarboxylic acid with the cellulosic fiber in the presence of acatalyst, such as sodium hypophosphite, at a temperature over 165° C.(see for example, U.S. Pat. Nos. 5,273,549; 5,137,537; 4,820,307;4,936,865; and 5,042,986 and European patent 0,427,316 B1, thedisclosures of which are incorporated by reference herein in theirentirety). Dialdehydes (for example glyoxal) are capable ofcross-linking cellulose hydroxyl groups via acetal bonds (see, forexample, U.S. Pat. Nos. 4,889,595; 4,472,167; 4,822,453; and 6,207,278B1, the disclosures of which are incorporated by reference herein intheir entirety).

One problem associated with the use of alkanepolycarboxylic acid is thatthe cellulosic fibers cross-linked thereby tend to lose theircross-linking upon storage, and revert to uncross-linked fibers.Consequently, the fibers lose their favorable mechanical properties thatwere obtained upon cross-linking. One attempt to solve this problem wasto use polymeric cross-linking agents. World Patent No. WO99/31312describes the use of a polymeric polycarboxylic acid having a molecularweight of more than 500 as a cross-linking agent for cellulosic fiber.U.S. Pat. No. 6,290,867 B1, the disclosure of which is incorporated byreference herein in its entirety, describes the use ofpolyhydroxyalkylurea having at least two-urea moieties as across-linking agent for cellulosic fibers. Although fiber cross-linkingwith polymeric polycarboxylic acid has been successful and impartsseveral advantages to cross-linked fiber, those skilled in the art havesought simple reagents and processes to produce cross-linked cellulosicfibers with reduced discoloration, knots, and knits.

Cellulosic fibers typically are cross-linked in the fluff form.Processes for making cross-linked fiber in the fluff form comprisedipping swollen or non-swollen fiber in an aqueous solution ofcross-linking agent, catalyst, and softener. The fiber so treated thenusually is cross-linked by heating it at elevated temperatures in theswollen state as described in U.S. Pat. No. 3,241,553, or in thecollapsed state after fluffing as described in U.S. Pat. No. 3,224,926,and European patent No. 0,427,316B1, the disclosures of each of whichare incorporated by reference herein in their entirety.

Cross-linking of fibers in the fluff form is believed to improvephysical and chemical properties of fibers in many ways, such asimproving the resiliency (in the dry and wet state), increasing theabsorbency, reducing wrinkling, and improving shrinkage resistance.Unfortunately, it has been found that such cross-linking, if carried outon a fiber in the sheet form, tends to create substantial problems inthe final product. These problems include severe fiber breakage andincreased amounts of knots and nits (hard fiber clumps). Thesedisadvantages render the cross-linked product completely unsuitable formany applications. Several approaches have been tried to overcome theseproblems, many of which have made the cross-linking even morecomplicated, time consuming, and costly (see, for example, U.S. Pat.Nos. 5,399,240; 4,204,054; and 3,434,918, the disclosures of which areincorporated by reference herein in their entirety).

The description herein of certain advantages and disadvantages of knowncross-linked cellulosic fibers, and methods of their preparation, is notintended to limit the scope of the present invention. Indeed, thepresent invention may include some or all of the methods and chemicalreagents described above without suffering from the same disadvantages.

SUMMARY OF THE INVENTION

In view of the problems associated with some of the known cross-linkingprocesses, there is a need to develop a catalyst-free cross-linkingagent that offers the cellulosic fiber with advantages that are seen nowwith conventional cross-linked fibers made using catalysts. There alsois a need to provide a simple, relatively inexpensive process forcross-linking cellulosic fibers in sheet and fluff form that producescross-linked cellulosic fibers that are substantially free of knots andnits. It will be appreciated, however, that knots are advantageous forsome applications, and accordingly, the present invention is not in anyway limited to producing cross-linked cellulosic fibers substantiallyfree of knots. The present invention desires to fulfill these needs andprovide further related advantages.

It therefore is a feature of an embodiment of the invention to providecross-linked cellulosic fibers that have improved acquisition rates,softness, resiliency, absorbent capacity, centrifuge retention capacity,free swell, and absorbency under load. It also is a feature of anembodiment of the invention to provide a method of cross-linking acellulosic fiber that is relatively simple and inexpensive.

In accordance with these and other features of embodiments of theinvention, there is provided a chemically cross-linked cellulosic fiberthat has a centrifuge retention capacity, as determined in accordancewith the testing protocol described herein, of less than about 0.48grams of a 0.9 wt. % saline solution per gram of fiber (hereinafter“g/g”), and less than about 0.60 g/g when the fiber is cross-linked withat least one acid aldehyde cross-linking agent. The chemicallycross-linked cellulosic fiber also preferably has desirable properties,such as a free swell of greater than about 10 g/g, an absorbent capacityof greater than about 9.0 g/g, an absorbency under load of greater thanabout 8.0 g/g, less than about 10% of knots, less than about 6.5% offines, an acquisition rate upon the third insult of less than about 10.0seconds, and an ISO brightness of greater than about 80%. Theseproperties may be achieved singly, or in various combinations with oneanother.

In accordance with an additional feature of an embodiment of theinvention, there is provided a method of making a cross-linkedcellulosic fiber that includes supplying a cross-linking agent to asheet of caustic treated cellulosic fibers, drying, and curing the sheetto provide a cross-linked fiber. Another suitable method includessupplying a cross-linking agent to a cellulose fiber in fluff form,drying, fluffing, and curing the fluff fiber to provide a cross-linkedfiber.

In accordance with another feature of an embodiment of the invention,there is provided a method of utilizing the chemically cross-linkedcellulosic fibers of the present invention in an absorbent core of anabsorbent article, and the absorbent core and absorbent articles madethereby. The present invention also provides a method for incorporatingthe cross-linked fiber and the absorbent core in an absorbent article.

The cross-linked fibers of the present invention preferably haveenhanced bulking characteristics, porosity and absorption, may besubstantially free of nits and knots, if desired, are substantially freeof discoloration, and have enhanced brightness. These and other objects,features, and advantages of the present invention will appear more fullyfrom the following detailed description of the preferred embodiments ofthe invention, and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an infrared spectrum (IR) of a cross-linked fiber ofthe present invention, whereby the fiber was treated with 10% glyoxylicacid, dried, and cured at 370° F. for 15 min.

FIG. 2 illustrates an IR of a cross-linked fiber of the presentinvention, whereby the fiber was treated with 45% glyoxylic acid, dried,and cured at 370° F. for 15 min.

FIGS. 3 a and 3 b are scanning electron microscope (SEM) photographs ofa cross-linked fiber of the present invention.

FIGS. 4 a and 4 b are SEM photographs of a cross-linked fiber of thepresent invention, taken at different magnifications.

FIGS. 5 a and 5 b are SEM photographs of a fiber of the presentinvention cross-linked with a combination of cross-linking agents.

FIGS. 6 a and 6 b are SEM photographs of cross-sections of across-linked fiber of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to chemically cross-linked cellulosicfibers and to a method of making them. The method comprisescross-linking of cellulosic fiber in sheet, roll, or fluff form with amultifunctional cross-linking agent.

As used herein, the terms “absorbent garment,” “absorbent article” orsimply “article” or “garment” refer to mechanisms that absorb andcontain body fluids and other body exudates. More specifically, theseterms refer to garments that are placed against or in proximity to thebody of a wearer to absorb and contain the various exudates dischargedfrom the body. A non-exhaustive list of examples of absorbent garmentsincludes diapers, diaper covers, disposable diapers, training pants,feminine hygiene products and adult incontinence products. Such garmentsmay be intended to be discarded or partially discarded after a singleuse (“disposable” garments). Such garments may comprise essentially asingle inseparable structure (“unitary” garments), or they may comprisereplaceable inserts or other interchangeable parts.

The present invention may be used with all of the foregoing classes ofabsorbent garments, without limitation, whether disposable or otherwise.Some of the embodiments described herein provide, as an exemplarystructure, a diaper for an infant, however this is not intended to limitthe claimed invention. The invention will be understood to encompass,without limitation, all classes and types of absorbent garments,including those described herein.

Throughout this description, the expressions “upper layer,” “lowerlayer,” “above” and “below,” which refer to the various componentsincluded in the absorbent material are used to describe the spatialrelationship between the respective components. The upper layer orcomponent “above” the other component need not always remain verticallyabove the core or component, and the lower layer or component “below”the other component need not always remain vertically below the core orcomponent. Other configurations are contemplated within the context ofthe present invention.

The term “component” can refer, but is not limited, to designatedselected regions, such as edges, corners, sides or the like; structuralmembers, such as elastic strips, absorbent pads, stretchable layers orpanels, layers of material, or the like.

Throughout this description, the term “disposed” and the expressions“disposed on,” “disposed above,” “disposed below,” “disposing on,”“disposed in,” “disposed between” and variations thereof are intended tomean that one element can be integral with another element, or that oneelement can be a separate structure bonded to or placed with or placednear another element. Thus, a component that is “disposed on” an elementof the absorbent garment can be formed or applied directly or indirectlyto a surface of the element, formed or applied between layers of amultiple layer element, formed or applied to a substrate that is placedwith or near the element, formed or applied within a layer of theelement or another substrate, or other variations or combinationsthereof.

Throughout this description, the terms “top sheet” and “back sheet”denote the relationship of these materials or layers with respect to theabsorbent core. It is understood that additional layers may be presentbetween the absorbent core and the top sheet and back sheet, and thatadditional layers and other materials may be present on the sideopposite the absorbent core from either the top sheet or the back sheet.

Throughout this description, the term “impregnated” insofar as itrelates to a cross-linking agent impregnated in a fiber, denotes anintimate mixture of cross-linking agents and cellulosic fiber, wherebythe cross-linking agent may be adhered to the fibers, adsorbed on thesurface of the fibers, or linked via chemical, hydrogen or other bonding(e.g., Van der Waals forces) to the fibers. Impregnated in the contextof the present invention does not necessarily mean that thecross-linking agent is physically disposed beneath the surface of thefibers.

Throughout this description, the expression “second” or “secondarycross-linking agent” denotes an additional cross-linking agent, and inno way is intended to mean that the second or secondary cross-linkingagent is present in an amount less than the first cross-linking agent.Indeed, a number of embodiments of the invention include those in whichthe secondary cross-linking agent is present in an amount greater thanthe first cross-linking agent. For example, a blend of cross-linkingagents can be used in the invention whereby the first cross-linkingagent is glyoxylic acid and is present in an amount of 20 wt %, and thesecondary cross-linking agent is citric acid and is present in an amountof 80 wt %.

The present invention concerns chemically cross-linked fibers that areuseful in absorbent articles, and in particular, that are useful informing absorbent cores or acquisition layers in the absorbent article.The particular construction of the absorbent article is not critical tothe present invention, and any absorbent article can benefit from thisinvention. Suitable absorbent garments are described, for example, inU.S. Pat. Nos. 5,281,207, and 6,068,620, the disclosures of each ofwhich are incorporated by reference herein in their entirety includingtheir respective drawings. Those skilled in the art will be capable ofutilizing the chemically cross-linked cellulosic fibers of the presentinvention in absorbent garments, cores, acquisition layers, and thelike, using the guidelines provided herein.

The cellulosic fibers cross-linked in accordance with the presentinvention, preferably possess characteristics that are desirable inabsorbent articles. For example, the cross-linked cellulosic fiberspreferably have a centrifuge retention capacity of less than about 0.48grams of synthetic saline per gram of fiber (hereinafter “g/g”), andless than about 0.60 g/g when cross-linked with an acid aldehydecross-linking agent. The chemically cross-linked cellulosic fiber alsohas desirable properties, such as a free swell of greater than about 10g/g, an absorbent capacity of greater than about 9.0 g/g, an absorbencyunder load of greater than about 8.0 g/g, less than about 10% of knots,less than about 6.5% of fines, an acquisition rate upon the third insult(or third insult strikethrough) of less than about 10.0 seconds, and anISO brightness value of greater than about 80%. The particularcharacteristics of the cross-linked cellulosic fibers of the inventionare determined in accordance with the procedures described in moredetail in the examples.

The centrifuge retention capacity measures the ability of the fiber toretain fluid against a centrifugal force. It is preferred that thefibers of the invention have a centrifuge retention capacity of lessthan about 0.48 g/g, when cross-linked with any cross-linking agent,more preferably, less than about 0.45 g/g, even more preferably lessthan 0.4 g/g, and most preferably less than about 0.39 g/g. Thecross-linked cellulosic fibers of the present invention can have acentrifuge retention capacity as low as about 0.37 g/g. It also ispreferred that the fibers of the invention have a centrifuge retentioncapacity of less than about 0.60 g/g, when cross-linked with an acidaldehyde cross-linking agent, more preferably, less than about 0.55 g/g,even more preferably less than 0.5 g/g, and most preferably less thanabout 0.43 g/g.

The free swell measures the ability of the fiber to absorb fluid withoutbeing subjected to a confining or restraining pressure. The free swellpreferably is determined by the Teabag method described herein. It ispreferred that the fibers of the invention have a free swell of morethan about 10 g/g, more preferably, more than about 13 g/g, even morepreferably more than 18 g/g, and most preferably more than about 22 g/g.The cross-linked cellulosic fibers of the present invention can have afree swell as high as about 29 g/g.

The absorbent capacity measures the capacity of the fiber to absorbfluid while under a confining or restraining pressure. It is preferredthat the fibers of the invention have an absorbent capacity of more thanabout 9.0 g/g, more preferably, greater than about 9.5 g/g, even morepreferably greater than about 10.5 g/g, and most preferably greater thanabout 11.0 g/g. The cross-linked cellulosic fibers of the presentinvention can have an absorbent capacity as high as about 16.0 g/g.

The absorbency under load measures the ability of the fiber to absorbfluid against a restraining or confining force over a given period oftime. It is preferred that the fibers of the invention have anabsorbency under load of greater than about 8.0 g/g, although it can beas low as 7.5 g/g, more preferably, greater than about 8.5 g/g, and mostpreferably, greater than about 9.0 g/g. The cross-linked cellulosicfibers of the present invention can have an absorbency under load of ashigh about 14.0 g/g.

The third insult strikethrough measures the ability of the fiber toacquire fluid, and is measured in terms of seconds. It is preferred thatthe fibers of the invention have a third insult strikethrough of lessthan about 10.0 seconds, more preferably, less than about 9.7 seconds,even more preferably less than 9.5 seconds, and most preferably lessthan about 9.0 seconds. The cross-linked cellulosic fibers of thepresent invention can have a third insult strikethrough of as low asabout 6.1 seconds.

The cross-linked fibers of the present invention preferably have lessthan about 10% of knots, more preferably less than about 5% knots, andmost preferably, less than about 3% knots. The cross-linked fibers ofthe present invention also preferably have less than about 6.5% offines, preferably less than about 6.3% fines, and most preferably, lessthan about 6.0% fines. Indeed, the cross-linked cellulosic fibers of thepresent invention can have fines as low as about 3.2%.

It also is preferred in the present invention, that the cross-linkedfibers have a dry bulk of at least about 12 cm³/g fiber, more preferablyat least about 12.5 cm³/g fiber, even more preferably at least about 13cm³/g fiber, and most preferably at least about 13.5 cm³/g fiber.

The ISO brightness measures the brightness of the fibers, and ismeasured using TAPPI methods T272 and T525. It is preferred that the ISObrightness of the cross-linked fibers of the present invention begreater than 80%, more preferably, greater than about 83%, even morepreferably greater than about 84.5%, and most preferably, greater thanabout 85%. It is preferred that the ISO brightness of the fibers reflectthe brightness of the fibers when formed into a pad.

Any cellulosic fibers can be used in the invention, so long as theyprovide the physical characteristics of the fibers described above.Suitable cellulosic fibers for use in forming the cross-linked fibers ofthe present invention include those primarily derived from wood pulp.Suitable wood pulp can be obtained from any of the conventional chemicalprocesses, such as the Kraft and sulfite processes. Preferred fibers arethose obtained from various soft wood pulp such as Southern pine, Whitepine, Caribbean pine, Western hemlock, various spruces, (e.g. SitkaSpruce), Douglas fir or mixtures and combinations thereof. Fibersobtained from hardwood pulp sources, such as gum, maple, oak,eucalyptus, poplar, beech, and aspen, or mixtures and combinationsthereof also can be used in the present invention. Other cellulosicfiber derived form cotton linter, bagasse, kemp, flax, and grass alsomay be used in the present invention. The fiber can be comprised of amixture of two or more of the foregoing cellulose pulp products.Particularly preferred fibers for use in forming the cross-linked fibersof the present invention are those derived from wood pulp prepared bythe Kraft and sulfite-pulping processes.

The cellulosic fibers can be derived from fibers in any of a variety offorms. For example, one aspect of the present invention contemplatesusing cellulosic fibers in sheet, roll, or fluff form. In another aspectof the invention, the fiber can be in a mat of non-woven material.Fibers in mat form are not necessarily rolled up in a roll form, andtypically have a density lower than fibers in the sheet form. In yetanother feature of an embodiment of the invention, the fiber can be usedin the wet or dry state. It is preferred in the present invention thatthe cellulosic fibers be employed in the dry state.

The fibers of the present invention preferably have a high surfacepurity of cellulose, but it is not necessarily required that thecellulosic fibers have a high cellulose bulk purity. It is preferredthat the cellulosic fiber be cross-linked in the sheet or roll form, andmore preferably, be a caustic treated fiber with “high cellulosepurity.” The high cellulose purity refers to the surface purity of thecellulosic fibers. Throughout this description, the expression “highcellulose purity” refers to pulp comprising at least about 65%,preferably at least 75%, and most preferably, at least about 90%α-cellulose.

The cellulosic fiber that is cross-linked in accordance with the presentinvention while in the fluff form can be any of wood pulp fibers orfiber from any other source described previously. In another aspect ofthe invention, the cross-linked fibers in the fluff form suitable foruse in the present invention include caustic treated fiber.

Caustic treatment can be carried out by any method known in the art,such as those described in Cellulose and Cellulose Derivatives, Vol. V,Part 1, Ott, Spurlin, and Grafllin, eds., Interscience Publisher (1954).Caustic treatment of pulp can be carried out by mixing pulp in anaqueous solution of alkali metal, such as sodium hydroxide, washing,neutralizing or washing and neutralizing, and optionally drying thepulp.

The caustics used in the caustic treatment serve to extract residualssuch as lignin and hemicellulose that may be left on the pulp after thepulping and bleaching processes. While not intending on being bound byany theory, the inventors believe that these residuals may beresponsible for knots, nits and discoloration in cross-linked fiberprepared by traditional cross-linking methods. In addition, treatmentwith caustic solution at specific concentrations is capable ofconverting cellulose from its native structure form, cellulose I, to amore thermodynamically stable and less crystalline form cellulose II.

Reagents suitable for caustic treatment include, but are not limited to,alkali metal hydroxides, such as sodium hydroxide, potassium hydroxide,calcium hydroxide, and rubidium hydroxide, lithium hydroxides, andbenzyltrimethylammonium hydroxides. Sodium hydroxyide is a particularlypreferred reagent for use in the caustic treatment to produce cellulosicfibers suitable for cross-linking in accordance with the presentinvention. The pulp of the invention preferably is treated with anaqueous solution containing from about 4 to about 30% by weight sodiumhydroxide, more preferably from about 6 to about 20%, and mostpreferably from about 12 to about 16% by weight, based on the weight ofthe solution. Caustic treatment may be performed during or afterbleaching, purification, and drying. Preferably, the caustic treatmentis carried out during the bleaching and/or drying process.

It is preferred in the present invention that the caustic treatment becarried out at or about room temperature. Those skilled in the art willbe capable of treating the fibers with caustic at a suitable temperatureto effect efficient cross-linking, using the guidelines provided herein.

After caustic extraction, the cellulose fiber can be of any purity, andpreferably is of high cellulose purity, containing more than 65% byweight, preferably more than 70%, and even more preferably at leastabout 80% by weight of α-cellulose. It is even more preferred that thecellulose fiber be comprised of at least about 90% by weight, morepreferably at least about 95% by weight, and most preferably at leastabout 97% by weight of α-cellulose.

Commercially available caustic extractive pulp suitable for use in thepresent invention include, for example, Porosanier-J-HP, available fromRayonier Performance Fibers Division (Jesup, Ga.), and Buckeye's HPZproducts, available from Buckeye Technologies (Perry, Fla.).

Cross-linking agents suitable for use in the present invention includeacid aldehydes. As used herein, the term “acid aldehyde” refers toorganic molecules having carboxylic acid and aldehyde functional groups,such as glyoxylic acid and succinic semialdehyde. Glyoxylic acid isparticularly preferred for use in the present invention.

In accordance with another feature of the invention, the inventionprovides chemically cross-linked fibers that are cross-linked with ablend of one or more cross-linking agents that include an acid aldehydeand an additional, or second, cross-linking agent. For this embodiment,any cross-linking agent can be used as the secondary cross-linkingagent, or additional one or more cross-linking agents. Suitableadditional cross-linking agents for use in the present invention includepolyepoxides that contain hydrophobic saturated, unsaturated, branchedand unbranched alkyls, such as 1,4-cyclohexanedimethanol diglycidylether, diglycidyl 1,2-cyclohexanedicrboxylate, N,N-diglycidylaniline,N,N-diglcidyl-4-glycidyloxyaniline, and diglycidyl1,2,3,4-tetrahydrophthalate and glycerol propoxylate triglycidyl ether.Various combinations of these cross-linking agents also may be used.

Other suitable additional cross-linking agents include C₂-C₉polycarboxylic acids, such as butantetracarbocylic acid, citric acid,itaconic acid, maleic acid, tartaric acid, and glutaric acid. Otherpreferred additional cross-linking agents include polycarbocxylic acidscommercially available as, for example, polyacrylic acid, polymaleicacid, polyitaconic acid, polyaspartic acid, and polymethacrylic acid.Particularly preferred combinations of cross-linking agents include ablend of glyoxylic acid and citric acid, a blend of glyoxylic acid andpolymaleic acid, and blends of glyoxylic acid, citric acid, andpolymaleic acid.

In one feature of an embodiment of the invention, the cross-linkingagent may be applied to the cellulose fiber in an aqueous solution.Preferably, the aqueous solution has a pH of from about 1 to about 5,more preferably from about 2 to about 3. The present inventors havediscovered that an aqueous solution of an acid aldehyde cross-linkingagent can be used as is without any adjacent or additional pH controlagent.

In another feature of an embodiment of the invention, a water insolubleadditional cross-linking agent, e.g., polyepoxide, may be used. Whensuch a water insoluble additional cross-linking agent is used, it ispreferred to add a minor amount of a surfactant (e.g., a few drops—lessthan 1% by weight), and the cross-linking agents then may be applied tothe fiber as a dispersion as opposed to an aqueous solution.

In general any type of surfactant capable of forming a dispersion withthe water insoluble additional cross-linking agent can be used. Suitablesurfactants include nonionic, anionic, or cationic surfactants, ormixtures and combinations of surfactants that are compatible with eachother. Preferably, the surfactant is selected from Triton X-100 (Rohmand Haas), Triton X405 (Rohm and Haas), sodium lauryl sulfate, laurylbromoethyl ammonium chloride, ethoxylated nonylphenols, polyoxyethylenealky ethers, polyoxyethylene alkyl ethers, and polyoxyethylene fattyacid ester.

The cellulosic fiber preferably is treated with an effective amount ofcross-linking agent to achieve the absorbent properties and physicalcharacteristics described herein. Generally, the fibers are treated withfrom about 1.0% to 10.0% by weight cross-linking agent, more preferablyfrom about 2% to 6%, and most preferably from about 3% to 5%. Theadditional cross-linking agent, if used, preferably is present in themixture in the amount of from about 5 to about 90% by weight, based onthe total weight of the mixture of cross-linking agents, morepreferably, the additional cross-linking agent is present in amount ofabout 20 to 60% by weight, based on the total weight of the mixture ofcross-linking agents. Optionally, the method of forming the cellulosicfiber in accordance with the invention includes a catalyst to speed upthe formation of an ester linkage between the hydroxyl groups of thecellulose and the cross-linking agent acid aldehyde groups. Thecross-linking reaction of the present invention, however, does notrequire a catalyst. To the extent that a catalyst is used, any catalystknown in the art that is capable of accelerating the formation of anester bond between hydroxyl groups and acid groups could be used in thepresent invention. Catalysts suitable for cross-linking include alkalimetal salts of phosphorous containing acids, such as alkali metalhypophosphites, alkali metal phosphites, alkali metal polyphosphonates,alkali metal phosphates, and alkali metal sulfonates. A particularlypreferred catalyst is sodium hypophosphite. A suitable weight ratio ofcatalyst to acid aldehyde is, for example, from 1:1 to 1:10, preferably1:2 to 1:6.

A catalyst also may be used to promote the reaction between polyepoxidesand cellulose hydroxyl groups, to the extent a cross-linking agentcontaining polyepoxide groups is used as an additional cross-linkingagent. When caustic treated fiber is used as the cellulosic fiber,however, a catalyst is not required. Any catalyst known in the art toaccelerate the formation of an ether bond or linkage between thehydroxyl groups of cellulose and an epoxide group can be used in thepresent invention. Preferably, the catalyst is a Lewis acid selectedfrom aluminum sulfate, magnesium sulfate, and any Lewis acid thatcontains at least a metal and a halogen, including, for example FeCl₃,AlCl₃, and MgCl₂. The catalyst can be applied to the fiber as a mixturewith the cross-linking agent(s), before the addition of thecross-linking agent(s), or after the addition of cross-linking agent(s)to cellulosic fiber.

Any method of applying the cross-linking agent(s) to the fiber can beused in carrying out the cross-linking method of the invention.Acceptable methods include, for example, spraying, dipping,impregnation, and the like. Preferably, the fiber is impregnated with anaqueous solution of cross-linking agent. Impregnation usually creates auniform distribution of cross-linking agent on the fiber and providesfor better penetration of cross-linking agent into the interior part ofthe fiber.

In one embodiment of the invention, a sheet of caustic treated fiber inthe roll form is conveyed through a treatment zone where a cross-linkingagent(s) is applied on both surfaces by conventional methods, such asspraying, rolling, dipping, knife coating, or other manners ofimpregnation. Preferred means of applying the aqueous solution ofcross-linking agent(s) to fiber in the roll form is by puddle press,size press, and blade coater.

Fibers in fluff, roll, or sheet form after treatment with thecross-linking agent(s) then are preferably dried and cured in a twostage process, and even more preferably, dried and cured in one stage.Such drying and curing is believed to remove water from the fiber,thereupon inducing the formation of an ester and an ether linkagebetween the hydroxyl groups of the fiber and the cross-linking agent(s),or “curing.” Curing usually is carried out in a forced draft ovenpreferably from about 300° F. to about 435° F., and more preferably fromabout 320° F. to about 425° F., and most preferably from about 360° F.to about 390° F. Curing preferably is carried out for a period of timethat permits complete fiber drying and efficient cross-linking. It ispreferred that the cellulosic fiber be cured for a period of timeranging from about 1 minute to about 25 minutes, and more preferablyfrom about 2 minutes to about 15 minutes, most preferably from about 3minutes to about 8 minutes.

In the case where the fibers are cross-linked in fluff form, theypreferably are defiberized by passing them through a hammermill or thelike, and then heated at elevated temperatures to promote cross-linking.The cellulosic fibers in fiber form can be dried at ambient temperaturesor at temperatures below the curing temperature. Preferably, thecellulosic fibers in fluff form are treated initially with thecross-linking agent(s), then fluffed, and then dried and cured atelevated temperatures in one procedure.

Where the cellulosic fibers to be treated with the cross-linkingagent(s) are in roll or sheet form, it is preferred that the fibers bedried and then cured, and more preferably dried and cured in oneprocedure. In one feature of an embodiment of the present invention, thefiber in sheet or roll form after having been treated with a solution ofcross-linking agent(s) is transported by a conveying device, such asbelt or a series of driven rollers, through a two-zone oven for dryingand curing. In another feature of an embodiment of the presentinvention, fibers in the non-woven-mat form after having been treatedwith a solution of cross-linking agent(s) preferably are transported bya conveying device, such as belt or a series of driven rollers, though ahammermill. The defiberized pulp produced by the hammermill thenpreferably is conveyed through a two-zone oven for drying and curing,preferably through a one step procedure in a one-zone oven for dryingand curing.

The present inventors have surprisingly found that fibers cross-linkedin accordance with the present invention with a mixture of cross-linkingagent(s), where the secondary or additional cross-linking agent is apolyepoxide, displayed significant increases in absorbency under load,softness, absorbent capacity, free swell, bulk, and brightness.

There are several advantages in the present invention for cross-linkingfibers in the sheet form besides being more economical. Fiberscross-linking in sheet form are expected to increase the potential forinter-fiber cross-linking which leads to “knots” and “nits” resulting inpoor performance in some applications. For instance, when a standardpurity fluff pulp, Rayfloc-J, is cross-linked in sheet form, the “knot”content increases substantially indicating increased deleteriousinter-fiber bonding. Examination of these “knots” recovered byclassification showed they contained true “nits” (hard fiber bundles).Surprisingly, it was found that high purity mercerized pulp cross-linkedin sheet or roll form actually yields far fewer knots and nits thancontrol pulps having conventional cellulose purity. Significantly,fibers in the sheet or roll from that were cross-linked in accordance tothe present invention were found to contain far fewer knots than acommercial individualized cross-linked fiber, like those produced by theWeyerhaeuser Company commonly referred to as HBA (for high-bulkadditive) and by Proctor & Gamble (“P&G”). It is preferred in thepresent invention that the cross-linked fibers have less than about 5%knots and nits, more preferably, less than about 4.5%, and mostpreferably less than about 4%.

Another benefit of using caustic treated cellulose pulp to producecross-linked pulp in fluff or sheet form according to the presentinvention is that the color forming bodies (hemicelluloses and lignins)are substantially removed, and the fiber is more stable to colorreversion at elevated temperature. Since cross-linking of celluloserequires high temperatures (typically around 185° C. for 10-15 minutes),this can lead to substantial discoloration with the conventional fluffpulps that are presently used. In product applications where pulpbrightness is an issue, the use of high purity cellulose pulp accordingto the present invention offers additional advantages.

Another benefit of the present invention is that cross-linked cellulosepulp sheets made in accordance with the present invention enjoy the sameor better performance characteristics as conventional individualizedcross-linked cellulose fibers, but avoid the processing problemsassociated with dusty individualized cross-linked fibers.

Cross-linked cellulosic fibers prepared in accordance with the presentinvention can be utilized, for example, as a bulking material, in themanufacture of high bulk specialty fiber applications that require goodabsorbency and porosity. The cross-linked fibers can be used, forexample, in non-woven, fluff absorbent applications. The fibers can beused independently, or preferably incorporated into other cellulosicfibers to form blends using conventional techniques, such as air layingtechniques. In an airlaid process, the fibers, alone or combined inblends with other fibers are blown onto a forming screen or drawn ontothe screen via a vacuum. Wet laid processes may also be used, combiningthe cross-linked fibers of the invention with other cellulosic fibers toform sheets or webs of blends.

The cross-linked fiber of the present invention can be incorporated intovarious absorbent articles preferably intended for body waste managementsuch as adult incontinent pads, feminine care products, and infantdiapers. The cross-linked fiber can be used as an acquisition layer inthe absorbent articles. Also it can be utilized in the absorbent core ofthe absorbent articles. Towels and wipes also may be made with thecross-linked fibers of the present invention, and other absorbentproducts such as filters. Accordingly, an additional feature of thepresent invention is to provide an absorbent core and an absorbentarticle that includes the chemically cross-linked fibers of the presentinvention.

The cross-linked fibers of the invention were incorporated into anacquisition layer of an absorbent article, and the absorbent article wastested by the Specific Absorption Rate Test (SART) test method, whereacquisition time of the fiber is important. The SART test method isdescribed in detail in the Examples. It was observed that the absorbentarticle that contained cross-linked fibers of the present inventionprovided results comparable to those obtained from using commercialcross-linked fiber, especially those cross-linked with citric acid orother polycarboxylic acids. The present inventors unexpectedlydiscovered that Porosanier-J-HP cross-linked in sheet form in accordancewith the present invention provided the best results, as shown by theSART method. Caustic treated fiber cross-linked in fluff form usingcurrently available approaches provided superior acquisition timecompared to those derived from conventional purity pulp used in currentindustrial practice (see Table 10, example 9 below).

As is known in the art, absorbent cores typically are prepared usingfluff pulp to wick the liquid, and an absorbent polymer (oftentimes asuperabsorbent polymer (SAP)) to store liquid. As noted previously, thecross-linked fibers of the present invention have high resiliency, highfree swell capacity, high absorbent capacity and absorbency under load,and low third insult strikethrough times. Furthermore, the cross-linkedfibers of the present invention are highly porous. Accordingly, thecross-linked fibers of the present invention can be used in combinationwith the SAP to prepare an absorbent composite (or core) having improvedporosity, bulk, resiliency, wicking, softness, absorbent capacity,absorbency under load, low third insult strikethrough, low centrifugeretention capacity, and the like. The absorbent composite could be usedas an absorbent core of the absorbent articles intended for body wastemanagement.

It is preferred in the present invention that the cross-linked fibers bepresent in the absorbent composite in an amount ranging from about 10 toabout 80% by weight, based on the total weight of the composite. Morepreferably, the cross-linked fibers are present in an absorbentcomposite from about 20 to about 60% by weight. A mixture ofconventional cellulosic fibers and cross-linked fibers of the presentinvention along with the SAP also can be used to make the absorbentcomposite. Preferably, the cross-linked fibers of the present inventionare present in the fiber mixture in an amount from about 1 to 70% byweight, based on the total weight of the fiber mixture, and morepreferably present in an amount from about 10 to about 40% by weight.Any conventional cellulosic fiber may be used in combination with thecross-linked fibers of the invention. Suitable additional conventionalcellulosic fibers include any of the wood fibers mentioned previously,caustic treated fibers, rayon, cotton linters, and mixtures andcombinations thereof.

Any suitable SAP can be used, or other absorbent material, to form theabsorbent composite, absorbent core, and absorbent article of thepresent invention. The SAP can be in the form of, for example, fiber,flakes, or granules, and preferably is capable of absorbing severaltimes its weight of saline (0.9% solution of NaCl in water) and/orblood. The SAP also preferably is capable of retaining the liquid whenit is subjected to a load. Non-limiting examples of superabsorbentpolymers applicable for use in the present invention include any SAPpresently available on the market, including, but not limited to,polyacrylate polymers, starch graft copolymers, cellulose graftcopolymers, and cross-linked carboxymethylcellulose derivatives, andmixtures and combinations thereof.

An absorbent composite made in accordance with the present inventionpreferably contains the SAP in an amount of from about 20 to about 60%by weight, based on the total weight of the composite, and morepreferably from about 30 to about 60% by weight. The absorbent polymermay be distributed throughout an absorbent composite within the voids inthe cross-linked fiber or the mixture of cross-linked fibers andcellulosic fibers. In another embodiment, the superabsorbent polymer isattached to the fiber via a binding agent that includes, for example, amaterial capable of cross-linking the SAP to the fiber via hydrogenbonding, (see, for example, U.S. Pat. No. 5,614,570, the disclosure ofwhich is incorporated by reference herein in its entirety).

A method of making an absorbent composite of the present invention mayinclude forming a pad of cross-linked fibers or a mixture cross-linkedfibers and cellulosic fibers, and incorporating particles ofsuperabsorbent polymer in the pad. The pad can be wet laid or airlaid.Preferably the pad is airlaid. It also is preferred that the SAP andcross-linked fibers, or a mixture of cross-linked fibers and cellulosicfibers, are air laid together.

Absorbent cores containing cross-linked fibers and superabsorbentpolymer preferably have dry densities of between about 0.1 g/cm³ and0.50 g/cm³, and more preferably from about 0.2/cm³ to 0.4 g/cm³. Theabsorbent core can be incorporated into a variety of absorbent articles,preferably those articles intended for body waste management, such asdiapers, training pants, adult incontinence products, feminine careproducts, and toweling (wet and dry wipes).

While not intending on being limited by any theory of operation, thereaction scheme shown below represents one of the possible mechanisms ofthe fiber reaction with a glyoxylic acid cross-linking agent. The schemeis provided for the purpose of illustrating, not limiting, thecross-linking reaction of the present invention.

As shown in the following scheme, the cross-linking reaction between thecellulosic fiber and glyoxylic acid is a self-catalyzed reaction, whichstarts at room temperature via the formation of a hemiacetal linkage.The formation of the hemiacetal is promoted by the intra- andinter-hydrogen bonding in the glyoxylic acid molecule (see the reactionscheme below). The hydrogen bonding increases the electrophilicity ofthe aldehyde carbon and makes it more susceptible to attack by thecellulose hydroxyl groups. As a result, the hemiacetal linkage forms atroom temperature. The formation of the hemiacetal linkage is believed tostabilize the glyoxylic acid from decomposing upon heating at hightemperature to complete the cross-linking.

In one embodiment, the cross-linked fiber and method of the presentinvention differs from conventional cross-linking methods whereby thecross-linking of the present invention begins at room temperature. Inanother embodiment, the cross-linking does not need a catalyst toproceed, and hence, can be considered a “self catalyzed process.” Inanother embodiment, the cross-linking agent is attached to thecellulosic fiber from three sites via a combination of ether and anester bonding. In another embodiment, after curing the cellulosic fiberis totally free from cross-linking agents, because the non-reactedcross-linking agent decomposes during the curing process.

The stability of the cross-links formed in cellulosic fiber of thepresent invention was examined by an aging process described below inexample 8. The cross-linked fiber of the invention showed little or nochange in bulk after heating it for about 20 h at 90° C. In addition,fiber stored at ambient temperature and humidity for over 3 monthsexhibited a bulk that remained unchanged during this period of time.

The infrared (IR) spectra of the cross-linked fiber treated in the sheetform with glyoxylic acid are shown in FIGS. 1 and 2. A Nicolet MAGNA 760FTIR Spectrometer (Madison, Wis., USA) with a Thunderdome was used tocollect the infrared spectrum data. IR spectrum shown in FIG. 1represents a fiber treated with 10% glyoxylic acid dried and cured at370° F. for 15 min. The IR spectrum shows only one carbonyl-stretchingband at 1741 cm⁻¹. The band could be assigned to an ester carbonylstretching.

The absence of other carbonyl stretching bands between 1720-1710 cm⁻¹rule out the presence of the carboxyl acid groups. In addition, theabsence of the C—H stretching vibration bands typically present in2830-2695 proves that the aldehyde functional groups are completelyreacted.

FIG. 2 illustrates the IR spectrum of a fiber treated with 45% glyoxylicacid, dried and cured at 370° F. for 15 min. The spectrum is similar tothat show in FIG. 1. Specifically, the spectrum of FIG. 2 shows thepresence of an ester functional group, and the absence of the carboxylicacid and aldehyde functional groups.

Scanning electron microscope S360 (Leica Cambridge Ltd., Cambridge,England) photographs of representative cross-linked fibers of thepresent invention obtained from cross-linking of fiber (16%), caustictreated at room temperature, and glyoxylic acid (5%) are illustrated inFIGS. 3A and 3B. The photographs were taken at 75× magnification forFIG. 3A, and 1000× magnification for FIG. 3B.

Scanning Electron Microscope (SEM) photographs of representativecross-linked fibers of the present invention obtained from cross-linkingof conventional fiber (Rayfloc-J) in the fluff from with glyoxylic acid(5%) are shown in FIGS. 4A and 4B. The SEM photographs were taken at 75×magnification for FIG. 4A, and at 350× magnification for FIG. 4B.

SEM photographs of representative cross-linked fibers of the presentinvention obtained from cross-linking of fiber (16%) caustic treated atabout room temperature with a mixture of glycidyl 1,2-cyclohexanedicarboxylate (1%) and glyoxylic acid (4%) are illustrated in FIGS. 5Aand 5B. The SEM photographs were taken at 75× magnification for FIG. 5A,and at 350× magnification for FIG. 5B.

An SEM photograph of the cross section of a representative cross-linkedfiber of the present invention obtained from cross-linking fiber (16%)caustic treated at about room temperature with 3% glyoxylic acid isshown in FIG. 6A. The SEM photograph shown in FIG. 6A) was taken at 350×magnification. An SEM photograph of the cross section of arepresentative cross-linked fiber of the present invention obtained fromcross-linking Rayfloc-J fiber with 3% glyoxylic acid is shown in FIG.6B. The SEM photograph shown in FIG. 6B was taken at 350× magnification.

As shown in these figures, the cross-linked fibers of the presentinvention are twisted and curled. The cross-linked fibers prepared fromcaustic treated fiber are round while those prepared from pulp withconventional purity are flat and ribbon-like. Furthermore, thecross-linked fibers of the present invention are highly porous.

To evaluate the various attributes of the present invention, severaltests were used to characterize the cross-linked fibers' performanceimprovements resulting from the presently described method.

The invention will be illustrated but not limited by the followingexamples.

EXAMPLES

The following test methods were used to measure and determine variousphysical characteristics of the inventive cross-linked cellulosicfibers.

Test Methods

The Teabag Method

The Teabag Method is a test method used to measure the absorbentcapacity under zero load, or “free swell” of the inventive cross-linkedfiber. In this test, 2.000 g (±0.001 g) of cross-linked fiber is placedinto a pre-weighed (±0.001 g) cloth teabag whereby the open end of thetea bag that contained the cross-linked fiber was sealed with an iron.The teabag and contents then was placed in a pan of 0.9% saline solutionand allowed to soak for 30 minutes. The teabag then was removed from thesolution, hanged on a drip rack, and allowed to drip dry for 10 minutes.The teabag and contents were weighed and the amount of solution retainedin the fibers was determined. A teabag containing no fibers was rununder similar conditions, and serves as a blank. The results are used tocalculate the amount of saline in grams retained per gram ofcross-linked fiber and are expressed as free swell in the units of g/g.The free swell is determined in accordance with the equation below:Free swell=[Weight of sample−(Weight of dry sample+Weight ofteabag+Weight of liquid absorbed by blank)]/Weight of dry sample.The Absorbency Test Method

The absorbency test method was used to determine the absorbency underload, absorbent capacity, and centrifuge retention capacity of thecross-linked fibers of the present invention. The absorbency test wascarried out in a one inch inside diameter plastic cylinder having a100-mesh metal screen adhering to the cylinder bottom “cell,” containinga plastic spacer disk having a 0.995 inch diameter and a weight of about4.4 g. In this test, the weight of the cell containing the spacer diskwas determined to the nearest 0.0001 g, and then the spacer was removedfrom the cylinder and about 0.35 g of cross-linked fiber having amoisture content within the range of from about 4% to about 8% by weightwere air-laid into the cylinder. The spacer disk then was inserted backinto the cylinder on the fiber, and the cylinder group was weighed tothe nearest 0.0001 g. The fiber in the cell was compressed with a loadof 4 psi for 60 seconds, the load then was removed and fiber pad wasallowed to equilibrate for 60 seconds. The pad thickness was measured,and the result was used to calculate the dry bulk of the cross-linkedfiber.

A load of 0.3 psi then was applied to the fiber pad by placing a 100 gweight on the top of the spacer disk, and the pad was allowed toequilibrate for 60 seconds, after which the pad thickness was measured.The cell and its contents then were hanged in a Petri dish containing asufficient amount of saline solution (0.9% by weight saline) to touchthe bottom of the cell. The cell was allowed to stand in the Petri dishfor 10 minutes, and the it was removed and hanged in another empty Petridish and allowed to drip for 30 seconds. While the pad still was underthe load, its thickness was measured. The 100 g weight then was removedand the weight of the cell and contents was determined. The weight ofthe saline solution absorbed per gram fiber then was determined andexpressed as the absorbency under load (g/g).

The absorbent capacity of the cross-linked fiber was determined in thesame manner as the test used to determine absorbency under load above,except that this experiment was carried out under a load of 0.01 psi.The results are used to determine the weight of the saline solutionabsorbed per gram fiber and expressed as the absorbent capacity (g/g).

The cell from the absorbent capacity experiment then was centrifuged for3 min at 1400 rpm (Centrifuge Model HN, International Equipment Co.,Needham HTS, USA), and weighed. The results obtained were used tocalculate the weight of saline solution retained per gram fiber, andexpressed as the centrifuge retention capacity (g/g).

Fiber Quality

Fiber quality evaluations were carried out on an Op Test Fiber QualityAnalyzer (Op Test Equipment Inc., Waterloo, Ontario, Canada) and FluffFiberization Measuring Instruments (Model 9010, Johnson Manufacturing,Inc., Appleton, Wis., USA).

Op Test Fiber Quality Analyzer is an optical instrument that has thecapability to measure average fiber length, kink, curl, and finescontent.

Fluff Fiberization Measuring Instrument is used to measure knits andfine contents of fiber. In this instrument, a sample of fiber in fluffform was continuously dispersed in an air stream. During dispersion,loose fibers passed through a 16 mesh screen (1.18 mm) and then througha 42 mesh (0.36 mm) screen. Pulp bundles (knots) which remained in thedispersion chamber and those that were trapped on the 42-mesh screenwere removed and weighed. The former are called “knots” and the latter“accepts.” The combined weight of these two was subtracted from theoriginal weight to determine the weight of fibers that passed throughthe 0.36 mm screen. These fibers were referred to as “fines.”

Example 1

This example illustrates a representative method for making caustictreated pulp and cross-linking it.

A sample of Rayfloc®-J-LD (never dried) was obtained as a 33.7% solidwet lap from a Rayonier mill at Jesup, Ga., and is an untreated southernpine Kraft pulp sold by Rayonier Performance Fibers Division, Jesup, Ga.and Fernandina Beach, Fla. for use in products requiring goodabsorbency, such as absorbent cores in diapers. A 70.0 g (dry weightbasis) sample was treated with an aqueous solution of 16% (w/w) sodiumhydroxide at a consistency of 3.5%. Treatment was carried out at roomtemperature for about 10 min, and excess NaOH was then removed bysuction filtration or centrifuge. The resulting caustic treated pulp waswashed with excess water, neutralized to a pH of 5.4 with acetic acidsolution (0.01 M) at a consistency of about 3.5%. The pulp then wasformed into a sheet (12×12 inch) and treated while in the wet state witha 4.5% aqueous solution of glyoxylic acid by dipping and pressing toafford a sheet having 5% glyoxylic acid on fiber. The treated sheet wasthen dried and cured at 370° F. for about 15 min. The cured sheet wasfiberized by feeding it through a Hammer Mill. The absorbent properties(free swell, absorbent capacity, absorbency under load, centrifugeretention) of the resulting fibers were then determined. Resultsobtained from sheets cross-linked in the same manner at different curingtimes are summarized in Table 1. Results summarized in Table 1 indicatethat, longer curing time is preferred in the present invention toprovide for complete cross-linking. TABLE 1 Absorbent properties offiber cross-linked with glyoxylic acid (5%) at different cure time: Curetemperature 370° F. Absorbent Centrifuge Curing Absorbency UnderCapacity Free Swell Retention Time (min) Load (g/g) (g/g) (g/g) (g/g) 159.0 10.6 21.0 0.37 10 8.9 10.0 20.0 0.46 5 10.0 11.1 25.0 0.60

Example 2

This example illustrates the effect of curing time on absorbentproperties of a representative cross-linked fiber formed in accordancewith the present invention.

Fibers were caustic treated and cross-linked with glyoxylic acid as inexample 1 above, except that the caustic treated sheet was dried in anoven at 60° C. before treatment with glyoxylic acid (2.6%) solution.Final percentage of glyoxylic acid on fiber was 3% by weight, based ondry sheet weight. The results are shown in Table 2 below. TABLE 2Absorbent properties of fiber cross-linked with glyoxylic acid (3%) atvarious curing time: Cure temperature 370° F. Centrifuge Cure timeAbsorbency under Absorbent Free Swell Retention (min) Load (g/g)capacity (g/g) (g/g) (g/g) —¹ 10.0 0.97 ² 8.6 10.2 21.5 0.72 3 8.5 9.922.0 0.51 5 8.4 9.9 22.4 0.51 10  8.5 10.1 22.0 0.50¹Mercerized pulp untreated with cross-linking agent glyoxylic acid.² No curing was carried out on this sheet, only drying.

The results summarized in Table 2 reveal that the absorbent propertiesof the cross-linked fibers depend on curing time. Increasing the curetime generally results in a cross-linked fiber having better absorbentproperties. In addition, the results in Table 2 reveal that partialcuring may occur at low temperature, since centrifuge retention ofcaustic treated pulp decreased from 1.0 g to 0.72 g upon treatment withglyoxylic acid and drying at 60° C.

Example 3

This example illustrates the effect of increasing the amount ofglyoxylic acid on absorbent properties of a representative cross-linkedfiber formed in accordance with the present invention.

The pulp used in this example was Porosanier-J, which is commerciallyavailable from the Rayonier mill at Jesup, Ga., and was obtained in rollform. Five sheets (12×12 inch), each weighing about 70.0 g (dry weightbase) were obtained from the roll. The sheets were treated with anaqueous solution of glyoxylic acid at room temperature and at variousconcentrations, and cured at 370° F. for 15 min. The results are shownin Table 3 below. TABLE 3 Absorbent properties of fiber cross-linkedwith various amount of glyoxylic acid: Cure temperature 370° F. for 15min (includes drying) Absorbency Centrifuge % of glyoxylic under LoadAbsorbent Free Swell Retention acid on fiber (0.3 psi) (g/g) Capacity(g/g) (g/g) (g/g) 8.0 8.6 10.1 19.4 0.39 5.4 9.0 10.6 21.0 0.37 4.0 9.310.5 22.7 0.39 3.0 8.6 10.2 21.0 0.43 2.0 8.3 9.80 20.8 0.46

The results summarized in Table 3 reveal that the highest absorbencyunder load and free swell were achieved with about 4% cross-linkingagent on fiber. Also, the results show that increasing the amount ofcross-linking agent decreases the centrifuge retention capacity of thefiber. However, increasing the amount of cross-linking agent on fiber toabout 8% did not show any substantial effect on fiber absorbentproperties.

Example 4

This example illustrates the effect of curing temperature on absorbentproperties of a representative cross-linked fiber formed in accordancewith the present invention.

The pulp used in this example was Porosanier-J, commercially availablefrom the Rayonier mill at Jesup, Ga., and obtained in the roll form.Five sheets (12×12 inch), each weighing about 70.0 g (dry weight base)were obtained from the roll. The sheets were treated with an aqueoussolution of glyoxylic acid at about room temperature to provide a finalpercentage of glyoxylic acid on fiber of about 4% by weight, based onthe sheet weight. The treated sheets then were cured at various curetemperatures for 15 min. Absorbent properties of cross-linked sheets asa function of cure temperature are evaluated and results are summarizedin Table 4. TABLE 4 Absorbent properties of fiber cross-linked withglyoxylic acid at various curing temperatures: Cure time 15 min Curingtemper- Absorbency Absorbent Free Centrifuge ature under Load capacitySwell Retention ° F. (g/g) (g/g) (g/g) (g/g) 300 8.5 9.6 20.3 0.62 3208.4 9.7 21.5 0.52 340 9.0 10.4 21.8 0.50 370 9.3 10.5 22.7 0.39 420 9.210.3 22.3 0.39

The results summarized in Table 4 reveal that absorbency under load,absorbent capacity, and free swell increase with increasing curetemperature, whereas centrifuge retention capacity decreases withincreasing cure temperature. These results indicate that cross-linkingefficiency increases with increasing the cure temperature. However,increasing the cure temperature from 370 to 420° F. did not provide anysubstantial change in fiber absorbent properties.

Example 5

This example illustrates a representative method for making cross-linkedfiber in the fluff from.

A sample of Rayfloc®-J-LD (never dried) was obtained as a 33.7% solidwet lap from Rayonier mill at Jesup, Ga. A 70.0 g (dry weight base)sample was treated with an aqueous solution of 4% (w/w) sodium hydroxideat a consistency of 3.5%. Treatment was carried out at room temperaturefor about 15 min, and excess NaOH was removed by suction filtration orcentrifuge. The resulting caustic treated pulp was washed with excesswater, neutralized to a pH of 5.4 with acetic acid solution (0.01 M) ata consistency of about 3.5%. The pulp was then treated while in the wetstate with a 2.2% aqueous solution of glyoxylic acid by dipping andpressing to afford a fiber having 4% by weight glyoxylic acid. Thetreated fiber was then dried at room temperature, defiberized by feedingit through a hammermill (Kamas Mill H01, Kamas Industries AB, Vellinge,Sweden), and cured at 370° F. Fiber absorbent properties and bulk werethen evaluated. Results obtained from fiber treated with variousconcentrations of sodium hydroxide and cross-linked in the same mannerare summarized in Tables 5 and 6. TABLE 5 Cross-linking of nonmercerized and mercerized Rayfloc-J in the fluff form at constant curingtemperature (370° F.). % of Curing Absorbency Absorbent Centrifugeaqueous time under Load capacity Free Swell Retention NaOH (min) (g/g)(g/g) (g/g) (g/g) — 5 13.8 15.8 25.0 0.55 — 10 14.0 15.8 29.0 0.51 4 512.7 14.5 25.0 0.56 4 10 13.6 16.0 27.5 0.55 8 5 11.7 13.3 25.4 0.53 810 12.2 14.0 29.0 0.48

TABLE 6 Cross-linked fiber in fluff form, dry and wet bulk: curetemperature 370° F., cure time 10 min % of aqueous Curing time NaOH(min) Dry Bulk cc/g Wet Bulk cc/g — 10 19.6 16.0 4 10 22.0 16.1 8 1020.2 15.7

Example 6

This representative example illustrates the effect of using a mixture ofcross-linking agents on the absorbent properties of cross-linked fibers.

The pulp used in this example was Porosanier-J, commercially availablefrom the Rayonier mill at Jesup, Ga., and obtained in the roll form.Four sheets (12×12 inch), each weighing about 70.0 g (dry weight basis)were obtained from the roll. The sheets were treated at about roomtemperature with an aqueous solution of a mixture of glyoxylic acid andvarious polyepoxide cross-linking agents at various concentrations inthe presence of an emulsifying agent Triton X-100 (0.1% of the totalweight of cross-linking agents). The treated fibers were cured at 370°F. for 15 min. The absorbent properties of cross-linked fibers wereevaluated and the results are summarized in Table 7 below. TABLE 7Absorbent properties of fiber cross-linked with a mixture cross-linkingagents: Cure temperature 370° F. (includes drying and curing)Cross-linking Absorbency Absorbent Centrifuge agent) and % under Loadcapacity Free Swell Retention on Fiber (0.3 psi) g/g g/g g/g g/gGlyoxylic acid 8.4 10.2 21.6 0.46 (3%) + PPGDGE (2%) Glyoxylic acid 8.410.0 20.0 0.48 (4%) + PPGDGE (1%) Glyoxylic acid 8.1 10.3 23.0 0.46(3%) + DG-1,2-CHDC (2%) Glyoxylic acid 7.8 9.7 21.5 0.42 (4%) +DG-1,2-CHDC (1%)

Example 7

This example reveals the percent of fines and knots in representativecross-linked fibers made in accordance with the present invention, whencompared to commercially available cross-linked fibers. TABLE 8 Finesand knots contents % of Cross- linking Cross-linking agent % of % ofFiber agent on fiber fines knots Porosanier 2.1 12.4 P & G 5.9 13.8(Pamper ® AL material) HBA 6.0 11.9 Porosanier Mixture of 4.2 4.4 1.5Fiber (sheet polymaleic form) acid (20%) and citric acid (80%)Cross-linked Glyoxylic 3.0 5.3 2.4 fiber acid (sheet form) Cross-linkedGlyoxylic 4.0 4.3 4.9 fiber acid (sheet form) Individualized Glyoxylic 55.3 23.6 cross- acid linked¹ fiber Individualized Glyoxylic 3 3.2 16.0cross- acid linked fiber² Cross-linked Glyoxylic 5.0 0.9 5.0 fiber acid(80%) (sheet form) and Diglycidyl 1,2-cyclohexane dicarboxylate (2%)¹Prepared from pulp with conventional purity.²Prepared from caustic (4%) treated fiber.

As indicated in Table 8, the percentage of knots in representativefibers cross-linked in sheet form using caustic treated pulp issignificantly lower than that for commercially available cross-linkedfiber and fiber cross-linked in fluff form in accordance with thepresent invention using pulp with conventional purity.

Example 8

This example describes the “aging” test method used to study theresistance of representative samples of cross-linked fiber made inaccordance with present invention to revert to uncross-linked fiber.Such reversion was observed in traditional cross-linked fiber made fromcross-linking fibers with alkane polycarboxylic acids, such as citricacid.

The aging test was carried out on two representative samples ofcross-linked fibers made in accordance with the present invention, asdescribed in Example 4 above. Each sample weighed about 2.000 g, thesamples were airlaid to pads each having a diameter of about 60.4 mm.One pad served as a blank, and the other was aged by heating it in anoven with a controlled humidity of 80% to about 85% at 90° C. for 20 hr.After the setting time, the sample pad was allowed to equilibrate in a50% humidity environment at room temperature for 24 hours. The two pads(sample and blank) then were compressed with a load of about 7.6 psi for60 seconds, the weights were removed, and the pads were allowed toequilibrate for 1 minute. The thickness of the pads were measured andbulk was determined.

The absorbent properties of blank and sample were determined by theabsorbency test method described above. The results are summarized inTable 9 below. TABLE 9 Absorbent properties of aged fiber Cross-linkedAbsorbency Absorbent Centrifuge Fiber (3% Dry Bulk Under Load capacityRetention glyoxylic acid) cc/g g/g g/g g/g Aged Sample 14.1 8.3 9.4 0.41Blank 14.0 8.6 10.2 0.43

The results summarized in Table 9 reveal that the bulk and centrifugeretention of cross-linked fiber remained unchanged after heating thefiber at elevated temperature for a long period of time. These resultsindicate that the cross-linkage in the fibers cross-linked in accordancewith the present invention are stable.

Example 9

This example provides the acquisition times for cross-linked fibers madein accordance with the present invention compared to commerciallyavailable cross-linked fibers. The fibers made in accordance with thepresent invention were prepared in accordance with Example 4 above.

The acquisition time was determined by the SART test method. The SARTtest method evaluates the performance of the cross-linked fibers as anacquisition layer in absorbent article. The test measures the timerequired for a dose of saline to be absorbed completely into theabsorbent article. The test is conducted on a sample of an absorbentarticle obtained from a commercially available diaper (Huggies, fromKimberly-Clark). The sample had a circular shape having a diameter of60.0 mm, usually cut from the center of the diaper core, and weighedabout 2.6 g (±0.2 g).

In this test, the acquisition layer of the sample was replaced with anairlaid pad made from the fiber of the present invention. The fiber padweighed about 0.7 g and was compressed with a load of a 7.6 psi forabout 60 seconds before it was used in the sample.

The sample was placed into the testing cell with the nonwoven side up.The testing cell consisted of a plastic base and a funnel cup (obtainedfrom Portsmouth Tool and Die Corp., Portsmouth, Va., USA). The base wasa plastic cylinder having an inside diameter of 60.0 mm that was used tohold the sample. The funnel cup was a plastic cylinder having a holewith a star shape, the outside diameter of which is 58 mm. The funnelcup was placed inside the plastic base on top of the sample, and a loadof about 0.6 psi having a donut shape was placed on top of the funnelcup.

The cell and its contents were placed on a leveled surface and dosedwith three successive insults, each being 9.0 ml of saline solution,(0.9% by weight), the time interval between doses being 20 min. Thedoses were added with a Master Flex Pump (Cole Parmer Instrument,Barrington, Ill., USA) to the funnel cup, and the time in secondsrequired for the saline solution of each dose to disappear from thefunnel cup was recorded and expressed as an acquisition time, orstrikethrough. The third insult strikethrough time is provided in Table10 below. TABLE 10 Liquid acquisition time for absorbent articlescontaining representative cross-linked fibers and commercial fibers. %of Cross- 3^(rd) Cross-linking linking Insult Fiber Agent agent (sec)Porosanier 21.0 P & G 8.9 (Pamper ® AL material) Porosanier Mixture of4.2 14.0 (sheet form) polymaleic acid (20%) and citric acid (80%)Cross-linked Glyoxylic acid 3.0 8.6 fiber (sheet form) IndividualizedGlyoxylic acid 5.0 6.6 cross-linked¹ fiber Individualized Glyoxylic acid3.0 6.1 cross-linked fiber² Cross-linked glyoxylic acid 5.0 9.0 fiber(sheet (80%) and form) Diglycidyl 1,2-cyclohexane dicarboxylate (20%)¹Prepared from pulp with conventional purity.²Prepared from caustic (4%) treated fiber.

As shown in Table 10, the third insult strikethrough times forrepresentative fibers formed in accordance with the present inventionwas lower than for commercially available cross-linked fiber. The liquidacquisition times for representative fiber formed in accordance with thepresent invention in fluff from were significantly less than forcommercially available cross-linked fiber.

Example 10

The fibers of Example 7 were tested for ISO Brightness in accordancewith TAPPI test methods T272 and T525. The results are summarized inTable 11 below: TABLE 11 ISO Brightness Fiber Cross-linking agent ISOBrightness Porosanier 91.4 P & G (Pamper ® AL 78.5 material)Cross-linked fiber Glyoxylic acid 87.8 (sheet form) Cross-linked fiberGlyoxylic acid 88.0 (sheet form) Individualized cross- Glyoxylic acid84.1 linked¹ fiber Individualized cross- Glyoxylic acid 83.9 linkedfiber² Cross-linked fiber Glyoxylic acid (80%) 89.1 (sheet form) andDiglycidyl 1,2-cyclohexane dicarboxylate (2%)¹Prepared from pulp with conventional purity.²Prepared from caustic (4%) treated fiber.

The results of Table 11 reveal that cellulose fibers cross-linked inaccordance with the present invention provide improved ISO brightness,when compared to conventional cross-linked fibers

While the invention has been described with reference to particularlypreferred embodiments and examples, those skilled in the art recognizethat various modifications may be made to the invention withoutdeparting from the spirit and scope thereof.

1. A method of making a cross-linked cellulosic fiber, comprising:treating cellulosic fibers with caustic; forming a wet laid sheet offibers; supplying a cross-linking agent to the sheet of fibers to form asheet impregnated with the cross-linking agent; and drying and curingthe cross-linking agent on the impregnated sheet of cellulosic fibers toform intra-fiber cross-links.
 2. The method of claim 1, wherein thefibers are treated with caustic which comprises treating cellulosicfibers with an aqueous solution of sodium hydroxide, washing the fibers,and neutralizing the treated fibers.
 3. The method of claim 2, whereintreating the cellulosic fibers with caustic is carried out in an aqueoussolution containing about 0.5% to about 40% by weight sodium hydroxide,based on the total weight of the solution.
 4. The method of claim 2,wherein treating the cellulosic fibers with caustic is carried out in anaqueous solution of sodium hydroxide at 2 to 10% consistency.
 5. Themethod of claim 2, wherein washing and neutralizing is carried out untilthe residual water has a pH of between 3 and
 8. 6. The method of claim 2wherein the cellulosic fibers are caustic treated in sheet or roll form.7. The method of claim 2, wherein the caustic treated cellulosic fiberis in the dry or wet state.
 8. The method of claim 2, wherein theα-cellulose content of the caustic treated cellulosic fibers is greaterthan 65%.
 9. The method of claim 8, wherein the α-cellulose content isat least 90%.
 10. The method of claim 1, wherein the cross-linking agentis one or more acid aldehyde organic molecules containing aldehyde andcarboxylic acid functional groups, and wherein the cross-linking agentis supplied to the caustic treated fiber in an aqueous solution.
 11. Themethod of claim 1, wherein a mixture of cross-linking agents aresupplied to the optionally caustic treated fiber, the mixture comprisinga primary cross-linking agent and a secondary cross-linking agent, andwherein the mixture is supplied to the fiber in an aqueous solution. 12.The method of claim 11, wherein the secondary cross-linking agent issupplied as an aqueous solution with a surfactant, the secondarycross-linking agent comprising a polyepoxide having a substituentselected from the group consisting of hydrogen; hydrophobic saturated,unsaturated, cyclic saturated, cyclic unsaturated, branched, unbranchedalkyl groups; and mixtures and combinations thereof.
 13. The method ofclaim 12, wherein the surfactant is selected from the group consistingof nonionic, anionic, cationic surfactant, or combinations and mixturesthereof.
 14. The method of claim 1, wherein the cross-linking agent issupplied to the cellulose fiber in an amount from about 0.5 to 10% byweight, based on the total weight of the fiber.
 15. The method of claim1, wherein the cross-linking agent is supplied to the cellulosic fibersin an amount from about 2 to 5% by weight, based on the total weight ofthe fiber.
 16. The method of claim 10, wherein the aqueous solution ofacid aldehyde cross-linking has a pH from about 1.5 to about 4.0. 17.The method of claim 12, wherein the surfactant is added in an amount offrom about 0.01 to 5% by weight, based on the total weight of thecross-linking agent.
 18. The method of claim 1, wherein drying andcuring is conducted at a temperature within the range of from about 320°F. to about 435° F.
 19. The method of claim 1, wherein drying and curingis conducted for about 3 minutes to about 15 minutes at temperatureswithin the range of from about 320° F. to about 435° F. 20-56.(canceled)
 57. The method of claim 1, wherein drying is conducted at atemperature within the range of from about 150 to about 300° F., andcuring is conducted at a temperature within the range of from about 320to about 435° F.
 58. The method of claim 1, wherein drying is conductedfor about 1 minute to about 10 minutes at temperatures within the rangeof from about 150 to about 300° F., and curing is conducted for about0.5 to about 5 minutes at temperatures within the range of from about320° F. to about 435° F.